For centuries, people have used various types of casks, barrels and tanks for storing liquids or solids. Today, vertical storage tanks range in size from hundreds to millions of gallons. Determining the volume of a liquid or solid being stored in these tanks has been a major problem. The two common methods of determining level in a tank filled with a liquid are using floats within the tank, or "dipping" the tank, i.e., to insert a graduated stick or measuring tape into the tank and seeing where the liquid level falls on the graduations. Once the level is known, the volume can be calculated using mathematics. Although dipping tanks is a simple procedure, it is also dangerous. Often, the tanks being measured have slippery tops and no catwalks. Measuring tank levels with floats is safer, but relies on mechanical devices within the tank that require maintenance. Maintaining equipment within a tank poses its own set of problems in that the tank has to be drained and entered for repairs.